An XML Schema Definition (XSD) is an instance of an Extensible Markup Language (XML) schema written in the W3C's XML Schema language. An XSD defines a type of XML document in terms of constraints upon what elements and attributes may appear, their relationship to each other, what types of data may be in them, and other things. It can be used with validation software in order to validate that an XML document falls within the constraints. XSDs were the first W3C-recommended XML schemas to provide a namespace- and datatype-aware alternative to using XML's native Document Type Definitions (DTDs). XML Schema Definition files usually have the filename extension “.xsd”.
Many different technologies use XML, and so benefit from the ability to validate XML using XSD's. For example, web services and web sites may use XML, and so can make use of XSDs.